The present invention relates to measuring fuel concentration in a liquid feed fuel cell. More particularly, the invention relates to a method and apparatus for indirectly measuring the concentration of fuel in an operating liquid feed fuel cell system.
In general, electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A solid polymer fuel cell is a specific type of fuel cell that employs a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. An electrocatalyst is employed to induce the desired electrochemical reactions at the electrodes. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. Solid polymer fuel cells typically operate in a range from about 40xc2x0 C. to about 150xc2x0 C.
A broad range of reactants has been contemplated for use in solid polymer fuel cells and such reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may, for example, be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture. An advantage of liquid feedstocks and/or fuels, such as methanol, particularly in non-stationary applications, is that they are relatively easy to store and handle. Further, fuel mixtures that react directly at the anode in a direct liquid feed fuel cell avoid the use of a reformer in the fuel cell system.
A liquid feed fuel cell is a type of solid polymer fuel cell that operates using at least one liquid reactant stream. Most typically, liquid feed fuel cells operate directly on an organic liquid fuel stream typically supplied as a fuel/water vapor or as an aqueous fuel solution. Typically, methanol is used as the fuel in a liquid feed fuel cell though other organic fuels may be used such as, for example, ethanol or dimethyl ether. When methanol is used, the liquid feed fuel cell is often referred to as a direct methanol fuel cell (DMFC). The methanol in the fuel stream is directly oxidized at the anode therein. There is often a problem in DMFCs with crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over typically reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, dilute solutions of methanol, for example, 5% methanol in water, are typically used as fuel streams. The fuel streams in DMFCs are usually recirculated in order to remove carbon dioxide, a by-product of the reaction at the anode, and to re-use the diluent and any unreacted fuel in the depleted fuel stream exiting the DMFC. Methanol is added to the circulating fuel stream before it re-enters the fuel cell in order to compensate for the amount consumed, thereby providing a fresh mixture at the desired methanol concentration. Since the amount of methanol consumed is variable (depending on the load, crossover, and other operating parameters), the methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor, and fresh methanol is admitted in accordance with the signal from the sensor.
Various types of sensors have been considered for purposes of measuring the concentration of methanol in aqueous solution and thus for use in a recirculating fuel stream in a liquid feed DMFC. For instance, electrochemical based sensors, which rely on the direct electro-oxidation of methanol in the fuel cell, may be considered. Advantages of electrochemical sensors include their simplicity, accuracy, fair reproducibility, and low-cost. However, electrochemical sensors suffer from degradation of the electrode reaction resulting in performance deterioration or failure over time.
Other types of sensors include capacitance devices that measure the change in dielectric constant of the fuel stream with methanol concentration. In theory, the larger the difference between the dielectric properties of two components of the fuel stream, the more precise the measurement can be. Unfortunately, the difference in dielectric constants for methanol-water systems is relatively small which may lead to misleading results or failure. Furthermore, the fuel in DMFCs is typically saturated with carbon dioxide, which may further exacerbate the difficulties in obtaining a precise measurement.
There are many factors to consider in developing a methanol sensor suitable for DMFCs. These factors include cost, size, simplicity, reliability, longevity, concentration range, and dynamic response. In particular, reliability and low cost should be addressed.
A liquid feed fuel cell system comprises a fuel cell stack having at least one fuel cell, a fuel delivery subsystem for providing a fuel stream to the fuel cell stack, and an oxidant delivery subsystem for providing an oxidant stream to the fuel cell stack. A method of measuring a fuel concentration in a fuel stream in such a fuel cell system comprises:
(a) measuring the temperature of the fuel stream entering the fuel cell stack;
(b) measuring a fuel cell stack temperature parameter indicative of the operating temperature of the fuel cell stack;
(c) measuring a current produced by the operating fuel cell system; and
(d) calculating the concentration of fuel in the fuel stream based on the above measurements and a predetermined calibration of the fuel cell system.
Typically, the fuel will be methanol and the fuel cell system will thus be a DMFC though other fuels may be used. The fuel cell stack temperature parameter may be, for example, the temperature of a reactant, either the oxidant or the fuel, leaving the fuel cell stack.
This method allows the indirect measurement of concentration of fuel in an operating fuel cell without the use of a dedicated sensor. Fuel concentration in the fuel stream can be expressed as a function of the current, stack temperature, and fuel stream temperature. Naturally, the fuel cell system should be previously calibrated which can be accomplished using conventional empirical modeling techniques.
In another embodiment, the fuel and oxidant stoichiometries may also be maintained substantially constant. By maintaining the reactant stoichiometries, the empirical modeling and subsequent calculations of fuel concentrations are simplified. Otherwise, it may be desirous to include the effect of reactant flow rates in the modeling and subsequent calculations.
In a further embodiment, the fuel cell system is operated by calculating the concentration of fuel in a fuel stream as above and then adjusting the concentration of fuel to maintain the fuel concentration within a desired fuel concentration range.
The apparatus for implementing the method of operating the fuel cell includes a fuel stream temperature sensor for monitoring the temperature of the fuel stream entering the fuel cell stack, a fuel cell stack temperature sensor, and a current sensor. A controller, in communication with these sensors, is then able to calculate the concentration of fuel in the fuel stream. As a result of the calculation, the controller, which is also in communication with a fuel injector, may adjust the rate fuel is added to the fuel stream and thereby maintain the fuel concentration at a desired level.